Coal Grinding

Coal grinding

To achieve good combustion and satisfactory flame formation, coal needs to be dried and ground to a proper degree of dryness and fineness. Drying of moisture in coal is achieved normally by ducting part of the kiln exhaust gas through the mill with inlet temperatures of up to 300°C. Inert kiln exhaust gases with oxygen content of about 3-5% are most suitable for the intended purpose due to high risk of fire/explosion in fine coal. However, the provision for inertization of coal mill circuit and fine coal bins (with CO2, N2 or CO2+N2 to replace O2 which promotes spontaneous ignition of fine coal) is strongly recommended to be available. Gas analysers and explosion vents are essentially provided in mill circuits to monitor the potential of fire/explosion and mitigate fire/explosion incident respectively. Drying and grinding are generally being done in either air swept ball mill or a vertical mill. The selection of mill system will depend mostly on the factors like initial capital cost, drying and grinding capacity required, cost of energy (power) etc.

Coal Fineness: It is understood generally that the finer we grind, the easy it is for burning. However, the fineness required will always be directed by where to fire and what type of coal it is and lastly the fineness will be dictated by the risk factor involved in finer grinding high volatile coals. The recommended fineness for coal verses volatile matter percentage is depicted in graph beside.

As understood from the above graph, the relationship between 90-micron and 200-micron residue is quite important as well. So, it is generally recommended to have 200-micron residue as low as possible, because coarse particles delays ignition, gives long flames in kiln (coating & ring issues), CO formation at kiln inlet, higher preheater exit temperatures (EGT). As a rule of thumb, the residue on 90-micron sieve should not be less than half of the volatile content for safety purpose.
ie. R90>=1/2. Volatile Content %.

Coal Moisture: The degree of drying, and therefore the required mill outlet temperature (from 65-80 0C) will depend upon the type of coal ground. Some residual moisture in fine coal is recommended (Graph below) to minimize the potential of spontaneous ignition of fine coal, which will again vary for different coal types as below:

Lignite Coals                           0.5-1.0 % by mass.
Bituminous Coals                   1.5-2.5 % by mass.
Anthracite Coals & Petcoke   0.5-1.0 % by mass.

While considering the safe mill outlet temperature, care should be taken to avoid the temperatures below dew point of mill outlet gases, so that the condensation inside the bag filter and consequent material jamming problems can be avoided.

Important Note: If you chose to use different types of coal (having different rank) simultaneously or use coal and Petcoke, remember to grind them separately as per above guidelines and feed them from different fine coal bins in required proportion to kiln and precalciner as required. 

Coal Grinding Operation Objectives and KPIs:
Highly energy intensive unit operation of coal grinding is intended to provide a fine coal as a fuel for calcination and clinkerization. Coal grinding operation is monitored for following parameters to ensure objectivity and economy of operation.

Product fineness (Residues) on 90-micron and 200-micron sieves.
Feed moisture, % and Product moisture, %
Production rate, tonnes/hour
Specific power consumption (SPC) kWh/tonne.

Specific wear rate consumption, g/t (checked during maintenance of mills)

Note: Proximate and ultimate analysis are generally provided by coal supplier. However, Proximate analysis, Determination of calorific value, Ash analysis, Hardgrove analysis and Abrasion analysis are done as and when required in plant laboratory or by a third party agency.

Coal Mill Operational parameters:

Mill Load (Kw or Amps).
Mill sound/filling % (in ball mills).
Mill Inlet Temperature (0C).
Mill Outlet Temperature (0C).
Gas flow through mill (m3/h) or mill fan power (kw)
Mill DP, or inlet/outlet draft (mmH2O).
Separator DP (mmH2O, mbar) and temperatures (0C).
Bag filter DP (mmH2O, mbar), Temperature (0C).

Coal Mill Safety parameters:

Position of Explosion vents.
Operational readiness of quick shutoff dampers.
Inertization section readiness (N2, CO2 pressure in bars)
Mill Inlet Temperature (0C).
Mill Outlet, bag filter outlet Temperature (0C).
O2 + CO Percent at bag filter outlet and in fine coal bins.
Bag filter hopper and fine coal bin temperature (0C).

Elements of Coal Grinding System:

Mill Feeding:
Consists of following activities
Coal Crusher: Generally, require when ball mill is used for grinding and raw coal size is on higher side(>25mm).
Conveying to Hoppers: Covered belt conveyors, horizontal or inclined are most suitable and commonly used for conveying.
Metal Detector and Magnetic Separator
Arrangement of metal detector and magnetic separator is integral part of feeding system in vertical roller mills and roller presses. Both are installed on mill feeding belt conveyor. Magnetic separator, separates out small metallic impurities from mill feed. While as metal detector signals the presence of metallic debris, which can damage the grinding path and give rise vibrations issues.
Mill Feeding Hoppers
Hoppers for coal, petcoke serve the purpose of providing a buffer storage for mill feed and a convenient arrangement for feeding to weigh feeders. Hoppers are generally designed to hold the requirements of one shift or more. Coal hoppers are generally steel hoppers with conical steep bottom (inclination >700), wide opening for discharge as possible to ensure mass flow of coal, mounted on load cells and/or equipped with level sensors to guide filling in auto mode. Dedusting bag filters needs to be installed at the top to vent air when material is fed to a hopper.
Mill Feeders:
Feeders for coal mill are generally installed directly under hoppers with rod gate in between. The feeders are generally 2 to 3 m long and discharge on to conveyor or feeding chute to mill. For coal mill feeding, table feeders, belt feeders, chain feeders and weigh feeders have been used. However, weigh feeders are the most commonly preferred to feed and report production counters.
Metering on Feeders:
Metering can be either direct (gravimetric) or indirect (Volumetric). In direct method of metering the material passes over a load cell installed in weigh feeder/apron feeder and the travel speed is monitored with installed tachometer. Weight and speed together determines the mass flow rate of material in metric tons per hour (t/hr). Alternatively, in some of the arrangements direct system consists of weigh feeder and its prefeeder. Feed rate is generally controlled with the speed of weigh feeder belt, which is driven by a variable speed drive. Feed rate is monitored and controlled by a control panel generally supplied with weigh feeder. Set points are passed to control panel from CCR by Operator. In indirect metering system feed rate is determined by measuring cross section of material and rate at which it flows and taking into account bulk density of material.

Calibration and Drop Test Facilities:
Provisions for drop test for calibration of weigh feeders are commonly available in cement plants to validate production figures. Although the weigh feeder calibration is not required frequently unless there is a disturbance in mechanical system due to various reason including maintenance. However, it is a common practice to validate feed rate through drop test periodically. Weigh feeders generally come with self-calibration devices. A simple way of 'self-calibration' is to have the hopper mounted on load cells, so that a weight loss for predefined time will be used for calibration purpose, and in this case calibration of hopper load cells needs to done at least once a year with standard weights.

Cold Air Locking at Mill Inlet:
This is very important for coal mill, as ambient air throttling the drying capacity of mill as well as increases the oxygen content of kiln gases making it riskier. Oxygen percentage of more than 12-14% is considered catalyst for fire/explosion risk. Rotary feeders (gravel gate), double flap valve are used to stop/minimize cold air leakage into mill system. Cold air leakage percentage can be determined by measuring oxygen percentage at inlet and outlet of the circuit element.

Coal Mill:
Most commonly used mills for coal grinding in cement plants are closed circuit air swept (Single chamber) ball mills and vertical mills.

Ball Mill:
Single chamber ball mills (with classifying liners and dam ring) with drying chamber and static or dynamic separator are commonly existing for coal grinding in cement plant for one or more kilns as per capacity. Ball mill is a cylinder rotating at about 70-80% of critical speed on two trunnions in white metal bearings or slide shoe bearings for large capacity mills. Grinding media consists balls of 3-4 sizes (60mm-30mm) in designed proportions with large sizes in feed end and small sizes in discharge end. About 27 to 35 % volume of mill is filled with grinding media. Equilibrium charge is that charge where compensation for wear can be done by balls of one size only usually the largest size in the compartment. Grinding media could be made of forged steel, cast steel or even cast iron. To economize grinding media consumption, presently grinding media used are high chrome steel balls. Mill shell is lined with lining plates to protect it from wear, high chrome steel liners are now commonly preferred to give longer life. Ground material is swept out of the mill by hot air /gas of significant velocity (5-6 m/s), through separator and coarse fraction is returned to the system for regrinding and fine material passed to bag filter for collection.

Vertical Roller Mills:
In Vertical Roller mill 2 - 4 rollers (lined with replaceable liners) turning on their axles press on a rotating grinding table (lined with replaceable liners) mounted on the yoke of a gear box. Pressure is exerted hydraulically. This mill also has a built in high efficiency separator above the rollers to reduce circulation loads and consequently reducing differential pressure across the mill. Feed material is directed onto the centre of the table and is thrown outward by rotation under the rollers by centrifugal action. Material gets partially ground and as it falls over the edge of the table, where it is picked up by hot gases, and is separated into coarse fraction falling back on grinding table and fine fraction is carried with hot gases to product collector.
The mill is started either with the rollers in lifted-up position, or with the hydro-pneumatic system at low pressure. In grinding mode, actual metal to metal contact should be prevented by limit switches or a mechanical stop and by consistent feed. In VRMs the material cycle time is usually less than a minute against several minutes for a ball mill or tube mill. Thus, control response should be accordingly faster. In case mill feed fails action should be taken within no more than 45 seconds or excessive vibration will cause mill shut-down. Moreover, the vertical mills are subject to vibrations if material is too dry to form a stable bed. Therefore, provision is made for controlled spray water inside the mill During mill operation magnetic separator and metal detector should be always functional to ensure to exclude tramp metal which can damage the grinding surfaces.

Primary Vertical Roller Mill Controls are:
Mill drive power or mill differential pressure to control mill feed rate.
Inlet gas temperature.
Outlet gas temperature.
Outlet gas flow.

Few countable salient Features of Vertical Mills

  • They are air swept mills, therefore has a higher drying capacity to handle an aggregate moisture of up to 20% in raw materials.
  • Space needed for vertical mill is much less than a closed circuit ball mill of same capacity.
  • As separator is integral part of the mill itself, the number of auxiliary equipments are less.
  • Total power consumption for grinding circuit as a whole is also less by about 30% compared to closed circuit ball mill in spite of higher fan power.
  • Feed size can be as large as 75 to 100 mm. Feed size can be as large as 5% of roller diameter.

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